This is definately the frame number for Metralla Mk2. The ´B´ represents ´Bastidor´ which is Spanish (Espanol) for frame. Then the first two digits of the serial number gives you the model number which is a ´model 23´. If you were to look at a parts book for a Metralla Mk2, you would find that all the parts that are specific to this model will begin with 23.0----. Hope this helps!